Spermatogonial stem cells (SSCs) are at the foundation of spermatogenesis and may have application for treating some cases of male infertility. High dose chemotherapy treatments for cancer and other non-malignant conditions can cause azoospermia, which may be due to a depletion of the SSC pool. If that is the case and the testicular environment (SSC niche) is functionally intact, then SSC transplantation may constitute a cell based therapy for restoring fertility in male cancer survivors. Theoretically, SSCs can be isolated via biopsy and cryopreserved prior to cancer treatment and then reintroduced into the testis after cure to regenerate spermatogenesis. The proof in principle for this approach is already established in several animal models, including recent progress from our lab in primates. Therefore, it is tempting to speculate that SSC transplantation might be translated to the human fertility clinic. This option may be particularly appropriate for prepubert boys who are not yet producing sperm and have no options to preserve their future fertility. With this in mind, several academic centers around the world, including our Fertility Preservation Program in Pittsburgh, are cryopreserving testicular tissues for boys in anticipation that SSCs can be used in the future to restore fertility. Pre-clinical studies are critically needed to demonstrate the feasibility of SSC transplantation in a model that is relevant to human anatomy and physiology as well as the target prepubertal patient population. In this application, we will treat prepubertal rhesus macaques with a clinically relevant high-dose alkylating chemotherapy regimen (BuCy2) that is expected to cause azoospermia. The SSC pool in nonhuman primate and human testes is comprised of Adark and Apale spermatogonia, but the relative regenerative potential of these morphologically distinct spermatogonial subtypes are not known. In Aim 1 we will evaluate and compare the stem cell properties of Adark and Apale spermatogonia, which will impact strategies to manipulate these cells for therapeutic purposes. Aims 2 will examine the effects of BuCy2 on the stem cell pool and somatic/endocrine environment (stem cell niche) of prepubertal testes. Aim 3 will model the clinical scenario of the prepubertal patient that is at high risk for azoospermia due to BuCy2 conditioning prior to hematopoietic stem cell transplantation. In that model, we will perform autologous SSC transplantation from frozen/thawed testicular cells. Fundamental knowledge generated in aims 1 and 2 could lead to novel SSC enrichment and niche therapy strategies that will be applied in Aim 3 to enhance spermatogenesis from endogenous or transplanted SSCs.